Base QI makes a microSD adapter that sits flush with the Surface book. As for displayport cables, YMMV across the pond, but I got a mini-displayport to displayport cable [CableMods] for something like $7.00 US. Just make sure it's certified 4k and you should be good to go.
Other than that, the new MS mouse [the rechargable one] is an awesome companion, and since I do a lot of travelling I also carry around a set of QC 35s [Bose noise cancelling headphones] Outside of that, a USB 3.0 hub w/ RJ45, a Samsung T3 or T5 external drive for overflow storage and/or additional program installation space [I could only get my hands on a 256, so this has been a LIFE SAVER]
Maybe eventually a D-brand skin to keep the chassis looking nice, but really there's not a LOT in the way of accessories that I wouldn't recommend for a traditional laptop/desktop [well, except the surface dial, but that's more of a gimick to scroll websites if you're not into the whole artist thing...